What about spyware software? It's a nightmare on the PC.
Just expanding a bit on Aptmunich's answer.
MacScan is a spyware checker that at one time was free. It disappeared from the net for years, then recently reappeared as a downloadable retail product. The website says it finds and removes spyware. It costs $24.95 U.S., but you can try it for nothing. I don't know how good it is or whether it has an Intel-native version.
Allume Systems, which makes the Stuffit compression program, sells
Internet Cleanup for $29.99 U.S. I know nothing about its track record, or whether it has a version for Intel inside. Anyway, I suspect that there are so few Mac spyware apps out there that any wouldn't be in its database anyway, if it uses a database.
Little Snitch is the Mac's answer to ZoneAlarm. I love it. It informs you whether any app (which would include spyware) is phoning home or otherwise is trying to gain access to the net. It doesn't check for incoming connections. For instance, Stuffit tries to phone home every time I run it (probably to check for updates). I set Little Snitch to block all Stuffit's attempts, and doing so stopped the warnings. But unlike ZoneAlarm's free version, Little Snitch costs $24.95 U.S.. I has a universal-binary version. It's well worth it.
Speaking of incoming connections, go to the System Preferences and click on Sharing, then the Firewall tab. Turn it on, then click on Advanced, at the bottom. Click on all three options. After an hour or so, check the log, available after clicking Advanced. (Maybe it has to be opened initially to turn it on.) There'll be a list of hundreds, if not thousands, of attempts to gain access to your machine. All of them will have been denied. (True on my machine, anyway.)
With all the Advanced options turned on, all ports are stealthed, according to the list of checks at
Sygate.
As Aptmunich said, there are
no Mac OS X viruses. Any application that claims to search for them could search only for the very limited number that were written for pre-OS X systems (some say there were only 80 to 90 over the decades those systems were current — and some predate the net, being propagated only through floppies), and those viruses were hardly a success, in any case.
The donation-ware application ClamXav,
available here, checks for Windows viruses that could be passed on to Windows machines (when I ran it the first time, it found one in an email attachment I received).
However, I've read that the always-running background-scan engine uses too much RAM or too many CPU cycles, or something. I don't know how true or how old that idea is. The backgrounder shouldn't have to be turned on, anyway, since it's a Mac, and you can set ClamXav to scan only email or cross-platform files, so it takes only a moment.
